This invention relates to the provision of optical coupling with an electro-optic transducer mounted on a single crystal substrate where at least part of that coupling is provided by an optical component located in a crystallographically etched groove or pit formed in the single crystal substrate. For the purpose of this specification the term crystallographic etching is defined to mean the type of etching in which a pit or groove is etched in a single crystal substrate and the depth of that pit or groove is determined by its width and by the fact that the etchant is employed under conditions in which it preferentially etches to specific crystal planes. Typically the optical component may be a ball lens mounted in an etch pit, or a graded index lens or the end of an optical fibre, mounted in an etched groove. A further example of an optical component that may be located in this way is an optical isolator. One of the advantages of this form of location of an optical component is the dimensional stability it is likely to afford in relation to the electro-optic transducer, and another is the precision of positioning it can afford.
An instance of the mounting of an optical fibre in a crystallographically etched Vee-groove in optical coupling relationship with an electro-optic transducer is described in GB 2,255,672A, to which attention is directed. In GB 2,255,672A there is described a method of making such an arrangement in which a photolithography process is used to define, and then anisotropically etch, a Vee-groove in a single crystal substrate. After the etching of the Vee-groove, further processing is employed to form a patterned electrically conductive ground plane covered with a dielectric layer of SiO.sub.2, still further processing is then employed to provide vias in the dielectric layer, and to cover it with an electrically conductive layer that is patterned to provide a number of interconnects. On one of these interconnects is mounted an electro-optic transducer in the form of a laser diode.
The preferred photolithographic processing includes the provision of a solder-wettable pad upon which to mount the transducer, the shape of this pad being such as to match that of the transducer so that the transducer can be bonded in alignment by solder-bump soldering. In solder-bump soldering one component is soldered to another, typically a component to a substrate, in a manner that provides automatic alignment of the two components through the effects of surface tension. For this purpose the facing surfaces of the two components to be soldered are provided with solder-wettable pads of matching configuration, and between these facing pads is located a predetermined volume of solder. When the solder is melted one of the components is allowed to move freely with respect to the other under the forces of surface tension provided by the molten solder. This freedom of movement is retained while the solder is caused to resolidify.